1. Field of the Invention
The invention relates to a semiconductor device on the surface of which a pad connected to a conductive wire and a protective coating are formed.
2. Description of the Related Art
A wire-bonding process is used to electrically connect a semiconductor device to an external circuit or to other semiconductor device. In a wire-bonding process, a wire that is connected at one end to an external circuit or to other semiconductor device is drawn onto a pad that is formed on the surface of the semiconductor device, and then the conductive wire is connected to the pad, and then the extension portion of the conductive wire is cut off. In the following description, such conductive wires will be simply referred to as “wires”.
When connecting each wire to each pad, the semiconductor device may be damaged by the wire. In view of this, some technologies for preventing such damage to the semiconductor device have been developed. Japanese Patent Application Publication No. 2001-298041 (JP-A-2001-298041) describes one such technologies. According to this publication, protective coatings are formed on a semiconductor device. Each protective coating is surrounded the corresponding pad formed on the semiconductor device. According to this structure, due to the difference between the level of the top face of the protective coating surrounding the pad and the level of the top face of the pad, once a pointing capillary, which is used to bond a wire to the pad, contacts the top face of the protective coating surrounding the pad, the pointing capillary is unable to approach the pad any further, and therefore the tip of the pointing capillary does not directly contact the pad. In this manner, the protective coating, which surrounds the pad, prevents damage to the pad. On the other hand, Japanese Patent Application Publication No. 05-136199 (JP-A-05-136199) describes a technology in which a protective coating is formed along an opening of an insulation layer covering the periphery of the pad such that the protective coating covers the surface of the insulation layer (Note that the insulation layer covers the surface of the semiconductor device and therefore it may be regarded as a portion of the semiconductor device and that the pad is exposed within the opening of the insulation layer). If the protective coating is not provided, the wire may contact an edge of the insulation layer that defines the opening when the pointing capillary presses the wire against the pad, and therefore there is a possibility that the wire may be welded to the pad by applying ultrasonic waves to the wire despite that the wire is in contact with the edge of the insulation layer. If ultrasonic waves are applied to the wire while it is in contact with the edge of the insulation layer, the insulation layer may be damaged. According to the technology described in JP-A-05-136199, therefore, in order to prevent the insulation layer covering the periphery of the pad from being damaged by such ultrasonic vibrations, the protective coating is formed along the opening of the insulation layer that determines the area in which the pad is exposed.
In the wire-bonding process, after the wire is connected to the pad, the extension portion of the wire is torn off by being pulled. At this time, the cut end of the extension portion of the wire (i.e., the end of the wire that is held by the pointing capillary to be connected to the next pad in the next wire-bonding process) may scratch and thereby damage the surface of the semiconductor device around the pad. The possibility of the cut end of the wire thus scratching and damaging the surface of the semiconductor device around the pad may be minimized or eliminated by forming a protective coating on the surface of the semiconductor device around the pad. However, such protective coatings in the related art cannot reliably prevent such scratching and damaging of the surface of the semiconductor device by the cut end of the wire.
In order to prevent scratching and damaging of the surface of the semiconductor device by the cut end of the wire, it is necessary to form a protective coating having a sufficient strength, that is, a relatively thick protective coating. If a thick protective coating is formed around the pad, the wire contacts the ridge line formed between the top face and the side faces of the protective coating. FIG. 7 illustrates an example case where a protective coating 70 is formed on a semiconductor device 402 at a position adjacent to the pad 16. The protective coating 70 has a thickness sufficient to prevent damage to the surface of the semiconductor device 402 by the cut end 14f of a wire 14. In this example, an acutely angled ridge line 72 is formed between the top face of the protective coating 70 and one side face of the protective coating 70, and an extension portion 14e of the wire 14 contacts the ridge line 72 in the wire-bonding process. Because the ridge line 72 is a very narrow portion of the protective coating 70, if the extension portion 14e of the wire 14 contacts the protective coating 70 at the ridge line 72, the protective coating 70 may be scraped off by the extension portion 14e of the wire 14. At this time, some pieces of the scraped protective coating 70 may attach the wire 14. If the next bonding process is performed using the wire 14 with said pieces of the protective coating 70 attached thereon, the bonding process may fail to be performed properly. Although such abrasion of the protective coating 70 may be prevented by increasing the hardness of the protective coating 70, if the protective coating 70 is made harder, it increases the possibility of delamination of the protective coating. Thus, the related-art protective coating 70 causes these problems when it protects the surface of the semiconductor device from being scratched and thus damaged by the cut end 14f of the wire 14.
Further, in the related art, in order to protect the surface of the semiconductor device from being scratched and thus damaged by the cut end of the wire, a pad is formed to cover the region where the cut end of the wire can damage the surface of the semiconductor device. However, it is often the case that useful semiconductor structures cannot be provided in the portion of the semiconductor substrate below the pad. According to the related art, further, the pad needs to be made relatively large to protect the surface of the semiconductor device from being damaged by the cut end of the wire, and therefore the area for accommodating semiconductor structures is reduced accordingly.